Generally, metal particles, such as zinc particles, iron particles, stainless steel particles, copper-zinc-nickel alloy particles, resin or glass particles plated with metal, corrode in the presence of water, oxygen or electrolyte ions. It is required that the metal particles are, therefore, carefully handled. For example, they are stored in a nitrogen blanket or an organic solvent in order to separate from corrosive conditions. If the metal particles corroded in a high corrosion rate, hydrogen gas which is difficult to deal with would be produced. Also, if they are formulated into a coating composition, they corrode by an invasion of water, oxygen or an electrolyte into a coating to result in a decline of the coatings.
In order to prevent the corrosion, it is proposed to allow the metal particles to contact with a chromate ion. The chromate ion has two functions. One is an oxidizer function whereby the chromate ion (CrO.sub.4.sup.2-) is reacted with a metal atom, such as Fe to form stable gamma-Fe.sub.2 O.sub.3, and the other is a deposition function whereby the chromate ion is reduced to form a chromium oxide, for example Cr.sub.2 O.sub.3 which is deposited on a metal surface. According to these two functions, a physical barrier film is formed on the metal surface to develop excellent corrosion preventive properties.
however, the chromium (VI) having high corrosion preventive properties is toxic to human body and therefore very limited in use by several regulations in Japan. Accordingly, many efforts have been paid to develop a corrosion preventive agent which is not toxic or has little toxicity to the human body. For example, phosphate materials, especially zinc phosphate, silica phosphate, aluminum condensed phosphate and the like are now being studied and some of them are commercially available. The phosphate materials, however, have only a deposition function whereby the phosphate materials are reacted with metal to produce a material protecting the metal substrate, and do not have an oxidizer function. The phosphate materials have low corrosion preventive properties in comparison with the above mentioned chromate ions.
The present inventors have proposed that the oxidizer function which was lacking in the phosphate materials could be compensated for by a soluble ion of vanadium (see Japanese Patent Application Ser. No. 204794/1986). The inventors further developed a novel surface treatment process for the metal particles.